An antenna substrate such as a planar antenna has a laminated structure. The laminated structure includes a strip line which includes a conductor plate and a dielectric substrate, and the strip line includes a signal line inside thereof. To feed a high frequency signal into the signal line in the strip line, a coaxial-strip line converter has been proposed. This converter couples the signal line in the strip line and an internal conductor of the coaxial connector. Specifically, a through-hole penetrating the laminated structure of the strip line is provided in the laminated structure. A conductive via is provided in the through-hole. The signal line inside the strip line is electrically coupled to an internal conductor of the coaxial connector through this via.
Herein, the vicinity of a central portion of the via (between one end and the other end) is coupled to the signal line, and the one end of the via is coupled to the internal conductor of the coaxial connector. On the other hand, the other end of the via not coupled to either of the signal line or the conductor. That is, the other end of the via is formed into an open stub. The presence of the open stub increases inconsistent losses.
If a difference in thermal expansion coefficient between the dielectric substrate and the via material is large, the volume expansion due to a temperature change thereof differ between the dielectric substrate and the via material. For this reason, the via is broken due to use over a long period of time, which increases transmission loss of the converter. That is, the reliability of the via degrades. This problem becomes conspicuous when the number of laminated layers (the number of dielectric substrates) of the laminated structure of the strip line is increased, and the aspect ratio of the through-hole (via) increases.
On the other hand, there is a method in which a multi-layered substrate is generated by a build-up construction method, and a via is provided partly inside the substrate. This method can prevent a problem from occurring with the open stub. However, in the build-up construction method, if, for example, PTFE (polytetrafluoroethylene) is used as a dielectric substrate, this makes manufacturing difficult and increases manufacturing costs.